Wednesday, September 25, 2013

Alison Gopnik -- Genes and Intelligence

We all notice that some people are smarter than others. You might naturally wonder how much these differences in intelligence depend on genes or upbringing. But that question, it turns out, is impossible to answer.

That's because changes in our environment can actually transform the relationship among our traits, our upbringing and our genes.

Okay, Genetic X Environmental interactions (genotype x environment = phenotype). But the amount of variance in a population that can be attributed to either genes or environment is not an "impossible" question. What she is avoiding saying right off the bat is that most of the variation in intelligence observed in people from middle-class (or better) backgrounds is attributable to genetic variation.

The textbook illustration of this is a dreadful disease called PKU. Some babies have a genetic mutation that makes them unable to process an amino acid in their food, and it leads to severe mental retardation. For centuries, PKU was incurable. Genetics determined whether someone suffered from the syndrome, which gave them a low IQ.

Then scientists discovered how PKU works. Now, we can immediately put babies with the mutation on a special diet. Whether a baby with PKU has a low IQ is now determined by the food they eat—by their environment.

We humans can figure out how our environment works and act to change it, as we did with PKU. So if you're trying to measure the relative influence of human nature and nurture, you have to consider not just the current environment but also all the possible environments that we can create.

Gotta love the PKU example. A great way to illustrate G x E interaction. However, it is one of the rarest causes of low intelligence (although, barring the dietary intervention, it will lead to mental retardation). Don't make the mistake of thinking that if you observe someone with low intelligence, they are a victim of PKU. The rate of babies born with PKU is about 1 in every 15,000 live births.

This doesn't just apply to obscure diseases. In the latest issue of Psychological Science, Timothy C. Bates of the University of Edinburgh and colleagues report a study of the relationship among genes, SES (socio-economic status, or how rich and educated you are) and IQ. They used statistics to analyze the differences between identical twins, who share all DNA, and fraternal twins, who share only some.

When psychologists first started studying twins, they found identical twins much more likely to have similar IQs than fraternal ones. They concluded that IQ was highly "heritable"—that is, due to genetic differences. But those were all high SES twins.Erik Turkheimer of the University of Virginia and his colleagues discovered that the picture was very different for poor, low-SES twins. For these children, there was very little difference between identical and fraternal twins: IQ was hardly heritable at all. Differences in the environment, like whether you lucked out with a good teacher, seemed to be much more important.

Identical twins are clones -- creepy, huh?

In the new study, the Bates team found this was even true when those children grew up. IQ was much less heritable for people who had grown up poor. This might seem paradoxical: After all, your DNA stays the same no matter how you are raised. The explanation is that IQ is influenced by education. Historically, absolute IQ scores have risen substantially as we've changed our environment so that more people go to school longer.

I probably would have summarized this research differently. Among twins from decent backgrounds, there is a high heritability for intelligence. Among very poor twins, there is little to no heritability. The awfulness of their environment wipes out whatever chance there was for higher intelligence. This is not the same as saying that better schools yield higher IQ. It does suggest that the genes associated with higher intelligence cannot flourish in poor environments. We are talking about IQ being suppressed by an impoverished environment. [Another news report here, quoting Eric Turkheimer from UVa., but getting itself confused with "nature v. nuture".]

Richer children have similarly good educational opportunities, so genetic differences among them become more apparent. And since richer children have more educational choice, they (or their parents) can choose environments that accentuate and amplify their particular skills. A child who has genetic abilities that make her just slightly better at math may be more likely to take a math class, so she becomes even better at math.

But for poor children, haphazard differences in educational opportunity swamp genetic differences. Ending up in a terrible school or one a bit better can make a big difference. And poor children have fewer opportunities to tailor their education to their particular strengths.

How your genes shape your intelligence depends on whether you live in a world with no schooling at all, a world where you need good luck to get a good education or a world with rich educational possibilities. If we could change the world for the PKU babies, we can change it for the next generation of poor children, too.

Here's the kicker. What happens when we give everyone the same environment? Everyone gets a two-family home in which at least one parent is employed and both are non-drug using, non-criminal high school graduates. Everyone goes to schools of the same quality. We will still see variations in intelligence (that is, some people will still be smarter than others). But then 100% of the variation in intelligence will be attributable to genetic variations in the population. Control for environment and the only explanation left for observed differences is genetic.

Footnote:

A version of this article appeared September 21, 2013, on page C2 in the U.S. edition of The Wall Street Journal, with the headline: Good Genes Only Get You So Far With Intelligence. It was released online with a different, more accurate headline: Poverty Can Trump a Winning Hand of Genes.